Developing highly-efficient power supplies, especially in combination with the ever-increasing constraints of high power density, is a continuing goal in power electronics. A switched-mode power converter is a frequently employed component of a power supply that converts an input voltage or current waveform into a specified output voltage or current waveform. There are several types of switched-mode converters including, for instance, a zeta converter.
A conventional zeta converter includes a power switch coupled to a controller, an isolation transformer, a storage capacitor, a rectifier and an output filter. The zeta converter generally operates as follows. The power switch conducts for a primary interval D to convey power from the input to the isolation transformer. During the primary interval D, energy is stored in the magnetizing inductance of the isolation transformer and flows from the storage capacitor to the output of the zeta converter. Then, for a complementary interval 1-D, the power switch is not conducting. The energy stored in the magnetizing inductance of the isolation transformer now flows to the storage capacitor.
The controller monitors the output voltage of the zeta converter and adjusts the duty cycle of the power switch to ultimately control the output voltage of the zeta converter. This degree of control provides a mechanism to maintain the output voltage at a relatively consistent level despite relative fluctuations in the input voltage and the load at the output.
In off-line power supply applications, a high power factor is frequently required. While a power factor of unity (i.e., 1.0) is the ultimate goal, a lesser power factor may, in some cases, be considered acceptable. Therefore, in applications employing the zeta converter, a power factor corrector (PFC) may be necessary to provide an acceptable power factor for the power supply.
An additional converter, such as a boost converter operating in a continuous conduction mode of operation or in a discontinuous conduction mode of operation (with the appropriate control circuitry), may serve as the PFC for the zeta converter. While the additional converter will provide an acceptable power factor for the power supply employing the zeta converter, there are several disadvantages. First, the power supply now employs two separate converter stages, each with an independent controller, to provide power conversion. As a result, the overall efficiency of the power supply is reduced because of the inherent inefficiencies associated with the two separate and distinctly controlled converter stages. Second, employing a boost converter, or any other power converter, to serve as a power factor corrector adds complexity to the power supply. With the goal of increasing the power density of the power supply, the additional converter detracts from, rather than facilitates, the simplicity of design of the power supply.
Accordingly, what is needed in the art is an single stage power converter that provides both a well-regulated output and a high power factor for a power supply employing the single stage power converter.